# duh- basic physics.  the speed of a falling object in the absence of air?

**URL:** <https://boards.straightdope.com/t/duh-basic-physics-the-speed-of-a-falling-object-in-the-absence-of-air/360999>\
**Category:** Factual Questions\
**Created:** [June 16, 2006, 9:56pm UTC](https://boards.straightdope.com/t/duh-basic-physics-the-speed-of-a-falling-object-in-the-absence-of-air/360999 "2006-06-16T21:56:05Z")\
**Posts on this page:** 6\
**Page:** 2

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**Author:** ![SCSimmons](https://avatars.discourse-cdn.com/v4/letter/s/e495f1/32.png) [@SCSimmons](https://boards.straightdope.com/u/SCSimmons)\
**Post date:** [June 17, 2006, 3:39pm UTC](https://boards.straightdope.com/t/duh-basic-physics-the-speed-of-a-falling-object-in-the-absence-of-air/360999/21 "2006-06-17T15:39:22Z")

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Good approximations, all. But everyone has neglected the change in gravitational acceleration over a 1 km distance! If the surface gravity at about 6000 km from the center of the earth is 9.81 m/s^2, the gravity at 6001 km will be about 9.80 m/s^2. So to get a fully accurate answer, you really have to do the calculus. 😛

Of course, to get a really, _really_ accurate answer, you need to do tensor calculus using the Einstein equation of general relativity, rather than mucking about with Newton’s Law of Universal Gravitation. You guys are so 18th century! 😉

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**Author:** ![CC](https://avatars.discourse-cdn.com/v4/letter/c/f1d935/32.png) [@CC](https://boards.straightdope.com/u/CC)\
**Post date:** [June 17, 2006, 4:44pm UTC](https://boards.straightdope.com/t/duh-basic-physics-the-speed-of-a-falling-object-in-the-absence-of-air/360999/22 "2006-06-17T16:44:28Z")

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So, does this come to around 320 mph?

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**Author:** ![zagloba](https://avatars.discourse-cdn.com/v4/letter/z/977dab/32.png) [@zagloba](https://boards.straightdope.com/u/zagloba)\
**Post date:** [June 17, 2006, 5:06pm UTC](https://boards.straightdope.com/t/duh-basic-physics-the-speed-of-a-falling-object-in-the-absence-of-air/360999/23 "2006-06-17T17:06:18Z")

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> [@Happy Clam](#):
>
> Oh, right, sorry.
> 
> Is the 1 kg part important? Since mass has no effect on the amount of force exerted on an object by gravity, in a vacuum it surely doesn’t matter whether the weight is 1kg or 1000kg when it hits the ground, just how long it’s been falling?

That’s not correct. The mass of an object _does_ affect the _force_ exerted on it by gravity. In fact the force is proportional to the mass. But the _acceleration_ of an object due to gravity doesn’t depend on mass because the mass cancels out:  
F = ma  
a = F/m

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**Author:** ![CC](https://avatars.discourse-cdn.com/v4/letter/c/f1d935/32.png) [@CC](https://boards.straightdope.com/u/CC)\
**Post date:** [June 17, 2006, 5:14pm UTC](https://boards.straightdope.com/t/duh-basic-physics-the-speed-of-a-falling-object-in-the-absence-of-air/360999/24 "2006-06-17T17:14:32Z")

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I say,

> [@CC](#):
>
> So, does this come to around 320 mph?

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**Author:** ![Rhubarb](https://avatars.discourse-cdn.com/v4/letter/r/59ef9b/32.png) [@Rhubarb](https://boards.straightdope.com/u/Rhubarb)\
**Post date:** [June 17, 2006, 8:04pm UTC](https://boards.straightdope.com/t/duh-basic-physics-the-speed-of-a-falling-object-in-the-absence-of-air/360999/25 "2006-06-17T20:04:18Z")

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> [@CC](#):
>
> So, does this come to around 320 mph?

psssst. See post #2. Plug and chug.

v=√(2as) = √(2_9.81_1000) = 140.07 m/s = 313.33 mph.

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**Author:** ![Bytegeist](https://avatars.discourse-cdn.com/v4/letter/b/9dc877/32.png) [@Bytegeist](https://boards.straightdope.com/u/Bytegeist)\
**Post date:** [June 17, 2006, 8:04pm UTC](https://boards.straightdope.com/t/duh-basic-physics-the-speed-of-a-falling-object-in-the-absence-of-air/360999/26 "2006-06-17T20:04:28Z")

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> [@CC](#):
>
> So, does this come to around 320 mph?

Yes.

I get 140 meters per second, which works out to 313 miles per hour.

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